Epidermal growth factor increases the labeling of phosphatidylinositol 3,4-bisphosphate in MA-10 Leydig tumor cells

Involvement of nitric oxide synthase in the mechanism of histamine-induced inhibition of Leydig cell steroidogenesis via histamine receptor subtypes in Sprague-Dawley rats

This study was conducted to shed light on the so far unexplored intracellular mechanisms underlying negative modulation of Leydig cell steroidogenesis by histamine (HA). Using the MA-10 cell line and highly purified rat Leydig cells as experimental models, we examined the effect of the amine on biochemical steps known to be modulated by HA or involved in LH/hCG action. In agreement with previous findings, HA at 10 microM showed a potent inhibitory effect on hCG-stimulated steroid synthesis, regardless of the gonadotropin concentration used. Moreover, HA decreased not only LH/hCG-induced cAMP production but also steroid synthesis stimulated by the permeable cAMP analog dibutyryl cAMP (db-cAMP). Considering the post-cAMP sites of HA action, it is shown herein that HA markedly inhibited db-cAMP-stimulated steroidogenic acute regulatory (STAR) protein expression, as well as steps catalyzed by P450-dependent enzymes, mainly the conversion of cholesterol to pregnenolone by cholesterol side-chain cleavage enzyme (CYP11A). The antisteroidogenic action of HA was blocked by addition of the phospholipase C (PLC) inhibitor U73122, and HA significantly augmented inositol triphosphate (IP3) production, suggesting a major role for the PLC/IP3 pathway in HA-induced inhibition of Leydig cell function. Finally, HA increased nitric oxide synthase (NOS) activity, and the NOS inhibitor NG-nitro-L-arginine methyl ester (L-NAME) markedly attenuated the effect of the amine on steroid synthesis. On the basis of our findings, HA antagonizes the gonadotropin action in Leydig cells at steps before and after cAMP formation. NOS activation is the main intracellular mechanism by which HA exerts its antisteroidogenic effects.

Dual role of histamine in modulation of Leydig cell steroidogenesis via HRH1 and HRH2 receptor subtypes

Although several reports indicate effects of histamine (HA) on female reproductive functions, scant literature exists to suggest a physiological role of HA in the male gonad. In the present study, we report a dual concentration-dependent effect of HA on steroidogenesis in MA-10 murine Leydig cells and purified rat Leydig cells. Although 1 nM HA can stimulate steroid production and significantly increase the response to LH/hCG in these cells, 10 microM HA exerts an inhibitory effect. We also provide confirming evidence for the existence of functional HRH1 and HRH2 receptors in both experimental models. The use of HRH1 and HRH2 selective agonists and antagonists led us to suggest that HRH2 activation would be largely responsible for stimulation of steroidogenesis, while HRH1 activation is required for inhibition of steroid synthesis. Our results regarding signal transduction pathways associated with these receptors indicate the coupling of HRH2 to the adenylate cyclase system through direct interaction with a Gs protein. Moreover, we show HRH1 activation mediates increases in inositol phosphate production, possibly due to coupling of this receptor to Gq protein and phospholipase C activation. The data compiled in this report clearly indicate that HA can modulate Leydig cell steroidogenesis in the testis and suggest a possible new physiological site of action for HA. Given that many drugs binding to HRH1, HRH2, or both, are widely prescribed for the treatment of diverse HA-related pathologies, it seems necessary to increase the knowledge regarding histaminergic regulation of testicular functions, to avoid possible unexpected side effects of such substances in the testis.

Mechanisms of epidermal growth factor signaling: regulation of steroid biosynthesis and the steroidogenic acute regulatory protein in mouse Leydig tumor cells

Steroid hormone biosynthesis in the adrenals and gonads is regulated by the steroidogenic acute regulatory (StAR) protein through its action in mediating the intramitochondrial transport of cholesterol. A role for epidermal growth factor (EGF) in modulating steroidogenesis has been previously determined, but the mechanism of its action remains unknown. The present investigation was designed to explore the potential mechanism of action of mouse EGF (mEGF) in the regulation of steroid biosynthesis and StAR protein expression in mLTC-1 mouse Leydig tumor cells. We show that treatment of mLTC-1 cells with mEGF significantly increased the levels of progesterone (P), StAR protein, and StAR mRNA in a time- and dose-dependent manner. The coordinate induction of P synthesis and StAR gene expression by mEGF was effectively inhibited by cycloheximide, indicating a requirement for de novo protein synthesis. Also, longer exposure of mLTC-1 cells to mEGF produced a marked decrease in LH-receptor mRNA expression. These effects of mEGF were exerted through high-affinity binding sites (K(d) approximately 0.53 nmol/L) in these cells. It was also determined that the arachidonic acid (especially lipoxygenase metabolites) and mitogen-activated protein kinase pathways were also involved in the mEGF-induced steroidogenic response. However, involvement of the latter pathway was further assessed in nonsteroidogenic COS-1 cells transfected with the Elk1 trans-reporting plasmids and resulted in a significant increase in luciferase activity in response to mEGF. Furthermore, deletion and mutational analyses demonstrated a predominant involvement of activator protein-1 in addition to the multiple mEGF responsive elements found within the 5'-flanking region (-151/-1 base pairs) of the mouse StAR gene. These findings provide novel insights into the mEGF-induced regulatory cascades associated with steroid synthesis and StAR protein expression in mouse Leydig cells.

Phosphoinositide 3-kinase C2alpha is activated upon smooth muscle cell migration and regulated by alpha(v)beta(3) integrin engagement

The involvement of phosphoinositide 3-kinase C2alpha in vascular smooth muscle cell migration was investigated. Products of phosphoinositide 3-kinase, phosphatidylinositol-3-phosphate, and phosphatidylinositol-3,4-bis-phosphate were increased upon smooth muscle cell migration but their synthesis was affected only partially by phosphoinositide 3-kinase inhibitors, wortmannin and LY-294002. Using specific antibody, we showed that the wortmannin/LY-294002 poorly sensitive phosphoinositide 3-kinase C2alpha is expressed in smooth muscle cells. Measurement of phosphoinositide 3-kinase C2alpha activity in vitro, after immunoprecipitation, clearly demonstrated its activation upon smooth muscle cell migration. Moreover, for the first time, phosphoinositide 3-kinase C2alpha was found to be differentially regulated by alpha(v)beta(3) and alpha(v)beta(5) integrin engagement. Finally, we have identified two new potential phosphoinositide 3-kinase C2alpha-binding proteins, p70 and p110, which both may be tyrosine phosphorylated. Thus, phosphoinositide 3-kinase C2alpha might represent a new regulatory pathway of cell migration downstream of integrin engagement.

Heregulin inhibits proliferation via ERKs and phosphatidyl-inositol 3-kinase activation but regulates urokinase plasminogen activator independently of these pathways in metastatic mammary tumor cells

Heregulin (HRG) and type I receptor tyrosine kinase (RTK) expression was investigated in the highly invasive and metastatic LM3 cell line, our previously described model of metastasis for mammary cancer (Bal de Kier Joffe et al. [1986] Invasion Metastasis 6:302-12; Urtreger et al. [1997] Int J Oncol 11:489-96). Although LM3 cells do not express HRG, they exhibit high levels of ErbB-2 and ErbB-3 as well as moderate expression of ErbB-4. Addition of exogenous HRGbeta1 resulted in inhibition of both proliferation and migration of LM3 cells. HRGbeta1 was also able to decrease the activity of urokinase-type plasminogen activator (uPA) and matrix metalloproteinase 9 (MMP-9), 2 key enzymes in the invasion and metastatic cascade. HRGbeta1 treatment of LM3 cells induced tyrosine phosphorylation of ErbB-2, ErbB-3 and ErbB-4 as well as the formation of ErbB-2/ErbB-3 and ErbB-2/ErbB-4 heterodimers. Assessment of the signaling pathways involved in HRGbeta1 action indicated that the addition of HRGbeta1 to LM3 cells resulted in activation of phosphatidylinositol 3- kinase (PI-3K) and in strong induction of the association of the p85 subunit of PI-3K with ErbB-3. HRGbeta1 also caused the rapid activation of ERK1/ERK2 and Stat3 and Stat5 (signal transducers and activators of transcription [STAT]). This is the first demonstration of the ability of HRGbeta1 to activate STATs in mammary tumor cells. Blockage of PI-3K activity with its chemical inhibitor wortmannin, or of MEK1/ERKs activity with PD98059, resulted in suppression of the ability of HRGbeta1 to inhibit LM3 cell growth. Notwithstanding the suppression of these 2 signaling pathways, HRGbeta1 still proved capable of inhibiting uPA activity. Therefore, our results provide evidence that signaling pathways involved in HRGbeta1-induced proliferation appear to be distinct from those involved in HRGbeta1 regulation of uPA, a protease that plays a pivotal role in invasion and metastasis.

Stereo-specific substrate recognition by phosphatidylinositol phosphate kinases is swapped by changing a single amino acid residue

Type I and type II phosphatidylinositol phosphate (PIP) kinases generate the lipid second messenger phosphatidylinositol (PtdIns) 4,5-bisphosphate and thus play fundamental roles in the regulation of many cellular processes. Although the two kinase families are highly homologous, they phosphorylate distinct substrates and are functionally non-redundant. Type I PIP kinases phosphorylate PtdIns 4-phosphate at the D-5 hydroxyl group and are consequently PtdIns 4-phosphate 5-kinases. By contrast, type II PIP kinases are PtdIns 5-phosphate 4-kinases that phosphorylate PtdIns 5-phosphate at the D-4 position. Type I PIP kinases, in addition, also phosphorylate other phosphoinositides in vitro and in vivo and thus have the potential to generate multiple lipid second messengers. To understand how these enzymes differentiate between stereoisomeric substrates, we used a site-directed mutagenesis approach. We show that a single amino acid substitution in the activation loop, A381E in IIbeta and the corresponding mutation E362A in Ibeta, is sufficient to swap substrate specificity between these PIP kinases. In addition to its role in substrate specificity, the type I activation loop is also key in subcellular targeting. The Ibeta(E362A) mutant and other mutants with reduced PtdIns 4-phosphate binding affinity were largely cytosolic when expressed in mammalian cells in contrast to wild-type Ibeta which targets to the plasma membrane. These results clearly establish the role of the activation loop in determining both signaling specificity and plasma membrane targeting of type I PIP kinases.

Differential regulation of phosphoinositide metabolism by alphaVbeta3 and alphaVbeta5 integrins upon smooth muscle cell migration

Smooth muscle cell migration is a key step of atherosclerosis and angiogenesis. We demonstrate that alpha(V)beta(3) and alpha(V)beta(5) integrins synergistically regulate smooth muscle cell migration onto vitronectin. Using an original haptotactic cell migration assay, we measured a strong stimulation of phosphoinositide metabolism in migrating vascular smooth muscle cells. Phosphatidic acid production and phosphoinositide 3-kinase IA activation were triggered only upon alpha(V)beta(3) engagement. Blockade of alpha(V)beta(3) engagement or phospholipase C activity resulted in a strong inhibition of smooth muscle cell spreading on vitronectin. By contrast, blockade of alpha(V)beta(5) reinforced elongation and polarization of cell shape. Moreover, Pyk2-associated tyrosine kinase and phosphoinositide 4-kinase activities measured in Pyk2 immunoprecipitates were stimulated upon cell migration. Blockade of either alpha(V)beta(3) or alpha(V)beta(5) function, as well as inhibition of phospholipase C activity, decreased both Pyk2-associated activities. We demonstrated that the Pyk2-associated phosphoinositide 4-kinase corresponded to the beta isoform. Our data point to the metabolism of phosphoinositides as a regulatory pathway for the differential roles played by alpha(V)beta(3) and alpha(V)beta(5) upon cell migration and identify the Pyk2-associated phosphoinositide 4-kinase beta as a common target for both integrins.

Regulation of phosphoinositide metabolism, Akt phosphorylation, and glucose transport by PTEN (phosphatase and tensin homolog deleted on chromosome 10) in 3T3-L1 adipocytes

To investigate the roles of PTEN (phosphatase and tensin homolog deleted on chromosome 10) in the regulation of 3-position phosphorylated phosphoinositide metabolism as well as insulin-induced Akt phosphorylation and glucose metabolism, wild-type PTEN and its phosphatase-dead mutant (C124S) with or without an N-terminal myristoylation tag were overexpressed in Sf-9 cells and 3T3-L1 adipocytes using baculovirus and adenovirus systems, respectively. When expressed in Sf-9 cells together with the p110alpha catalytic subunit of phosphoinositide 3-kinase, myristoylated PTEN markedly reduced the accumulations of both phosphatidylinositol 3,4-bisphosphate and phosphatidylinositol 3,4,5-trisphosphate induced by p110alpha. In contrast, overexpression of the C124S mutants apparently increased these accumulations. In 3T3-L1 adipocytes, insulin-induced accumulations of phosphatidylinositol 3,4-bisphosphate and phosphatidylinositol 3,4,5-trisphosphate were markedly suppressed by overexpression of wild-type PTEN with the N-terminal myristoylation tag, but not by that without the tag. On the contrary, the C124S mutants of PTEN enhanced insulin-induced accumulations of phosphatidylinositol 3,4-bisphosphate and phosphatidylinositol 3,4,5-trisphosphate. Interestingly, the phosphorylation level of Akt at Thr308 (Akt2 at Thr309), but not at Ser473 (Akt2 at Ser474), was revealed to correlate well with the accumulation of phosphatidylinositol 3,4,5-trisphosphate modified by overexpression of these PTEN proteins. Finally, insulin-induced increases in glucose transport activity were significantly inhibited by the overexpression of myristoylated wild-type PTEN, but were not enhanced by expression of the C124S mutant of PTEN. Therefore, in conclusion, 1) PTEN dephosphorylates both phosphatidylinositol 3,4-bisphosphate and phosphatidylinositol 3,4,5-trisphosphate in vivo, and the C124S mutants interrupt endogenous PTEN activity in a dominant-negative manner. 2) The membrane targeting process of PTEN may be important for exerting its function. 3) Phosphorylations of Thr309 and Ser474 of Akt2 are regulated differently, and the former is regulated very sensitively by the function of PTEN. 4) The phosphorylation level of Ser474, but not that of Thr309, in Akt2 correlates well with insulin-stimulated glucose transport activity in 3T3-L1 adipocytes. 5) The activity of endogenous PTEN may not play a major role in the regulation of glucose transport activity in 3T3-L1 adipocytes.

Conformation, localization, and integrin binding of talin depend on its interaction with phosphoinositides

Talin is a structural component of focal adhesion sites and is thought to be engaged in multiple protein interactions at the cytoplasmic face of cell/matrix contacts. Talin is a major link between integrin and the actin cytoskeleton and was shown to play an important role in focal adhesion assembly. Consistent with the view that talin must be activated at these sites, we found that phosphatidylinositol 4-monophosphate and phosphatidylinositol 4,5-bisphosphate (PI4,5P(2)) bound to talin in cells in suspension or at early stages of adhesion, respectively. When phosphoinositides were associated with phospholipid bilayer, talin/phosphoinositide association was restricted to PI4,5P(2). This association led to a conformational change of the protein. Moreover, the interaction between integrin and talin was greatly enhanced by PI4,5P(2)-induced talin activation. Finally, sequestration of PI4,5P(2) by a specific pleckstrin homology domain confirms that PI4,5P(2) is necessary for proper membrane localization of talin and that this localization is essential for the maintenance of focal adhesions. Our results support a model in which PI4,5P(2) exposes the integrin-binding site on talin. We propose that PI4,5P(2)-dependent signaling modulates assembly of focal adhesions by regulating integrin-talin complexes. These results demonstrate that activation of the integrin-binding activity of talin requires not only integrin engagement to the extracellular matrix but also the binding of PI4,5P(2) to talin, suggesting a possible role of lipid metabolism in organizing the sequential assembly of focal adhesion components.

Natural biology of polyomavirus middle T antigen

"It has been commented by someone that 'polyoma' is an adjective composed of a prefix and suffix, with no root between--a meatless linguistic sandwich" (C. J. Dawe). The very name "polyomavirus" is a vague mantel: a name given before our understanding of these viral agents was clear but implying a clear tumor life-style, as noted by the late C. J. Dawe. However, polyomavirus are not by nature tumor-inducing agents. Since it is the purpose of this review to consider the natural function of middle T antigen (MT), encoded by one of the seemingly crucial transforming genes of polyomavirus, we will reconsider and redefine the virus and its MT gene in the context of its natural biology and function. This review was motivated by our recent in vivo analysis of MT function. Using intranasal inoculation of adult SCID mice, we have shown that polyomavirus can replicate with an MT lacking all functions associated with transformation to similar levels to wild-type virus. These observations, along with an almost indistinguishable replication of all MT mutants with respect to wild-type viruses in adult competent mice, illustrate that MT can have a play subtle role in acute replication and persistence. The most notable effect of MT mutants was in infections of newborns, indicating that polyomavirus may be highly adapted to replication in newborn lungs. It is from this context that our current understanding of this well-studied virus and gene is presented.

SAC1-like domains of yeast SAC1, INP52, and INP53 and of human synaptojanin encode polyphosphoinositide phosphatases

The SAC1 gene product has been implicated in the regulation of actin cytoskeleton, secretion from the Golgi, and microsomal ATP transport; yet its function is unknown. Within SAC1 is an evolutionarily conserved 300-amino acid region, designated a SAC1-like domain, that is also present at the amino termini of the inositol polyphosphate 5-phosphatases, mammalian synaptojanin, and certain yeast INP5 gene products. Here we report that SAC1-like domains have intrinsic enzymatic activity that defines a new class of polyphosphoinositide phosphatase (PPIPase). Purified recombinant SAC1-like domains convert yeast lipids phosphatidylinositol (PI) 3-phosphate, PI 4-phosphate, and PI 3,5-bisphosphate to PI, whereas PI 4,5-bisphosphate is not a substrate. Yeast lacking Sac1p exhibit 10-, 2.5-, and 2-fold increases in the cellular levels of PI 4-phosphate, PI 3,5-bisphosphate, and PI 3-phosphate, respectively. The 5-phosphatase domains of synaptojanin, Inp52p, and Inp53p are also catalytic, thus representing the first examples of an inositol signaling protein with two distinct lipid phosphatase active sites within a single polypeptide chain. Together, our data provide a long sought mechanism as to how defects in Sac1p overcome certain actin mutants and bypass the requirement for yeast phosphatidylinositol/phosphatidylcholine transfer protein, Sec14p. We demonstrate that PPIPase activity is a key regulator of membrane trafficking and actin cytoskeleton organization and suggest signaling roles for phosphoinositides other than PI 4,5-bisphosphate in these processes. Additionally, the tethering of PPIPase and 5-phosphatase activities indicate a novel mechanism by which concerted phosphoinositide hydrolysis participates in membrane trafficking.

Rac homologues and compartmentalized phosphatidylinositol 4, 5-bisphosphate act in a common pathway to regulate polar pollen tube growth

Pollen tube cells elongate based on actin- dependent targeted secretion at the tip. Rho family small GTPases have been implicated in the regulation of related processes in animal and yeast cells. We have functionally characterized Rac type Rho family proteins that are expressed in growing pollen tubes. Expression of dominant negative Rac inhibited pollen tube elongation, whereas expression of constitutive active Rac induced depolarized growth. Pollen tube Rac was found to accumulate at the tip plasma membrane and to physically associate with a phosphatidylinositol monophosphate kinase (PtdIns P-K) activity. Phosphatidylinositol 4, 5-bisphosphate (PtdIns 4, 5-P2), the product of PtdIns P-Ks, showed a similar intracellular localization as Rac. Expression of the pleckstrin homology (PH)-domain of phospholipase C (PLC)-delta1, which binds specifically to PtdIns 4, 5-P2, inhibited pollen tube elongation. These results indicate that Rac and PtdIns 4, 5-P2 act in a common pathway to control polar pollen tube growth and provide direct evidence for a function of PtdIns 4, 5-P2 compartmentalization in the regulation of this process.

Peroxynitrite affects Ca2+ transport in Trypanosoma cruzi

Macrophages play an important role against Trypanosoma cruzi infection, via superoxide, nitric oxide, and peroxynitrite production. Peroxynitrite has been shown to be highly cytotoxic against Trypanosoma cruzi epimastigotes. Calcium is involved in many vital functions of the parasites, being its intracellular concentration governed by several transport systems, involving mitochondrial and non-mitochondrial compartments. In this paper, we report the effect of peroxynitrite on the calcium uptake systems, as studied by digitonin-permeabilized trypanosomes in the presence of arsenazo III. Peroxynitrite, at biologically relevant concentrations produced within phagosomes (250-750 microM), inhibited calcium uptake in a dose-dependent manner. Peroxynitrite decreased the mitochondrial membrane potential obtained in the presence of tetramethyl-p-phenylenediamine (TMPD)/ascorbate. In addition, a decrease of the non-mitochondrial Ca(2+)-uptake, concomitant with the inactivation of a Ca(2+)-dependent ATPase activity, was observed. HPLC analyses of the cellular adenine nucleotide pool showed a time-dependent decrease of ATP content and energy charge of the parasite; however this drop in ATP levels was significantly delayed with respect to decrease of the ATP-dependent Ca(2+)-transport. We conclude that the disruption of calcium homeostasis by peroxynitrite may contribute to the observed cytotoxic effects of macrophages against T. cruzi.

Lipid products of phosphoinositide 3-kinase and phosphatidylinositol 4',5'-bisphosphate are both required for ADP-dependent platelet spreading

We have shown previously that ADP released upon platelet adhesion mediated by alphaIIb beta3 integrin triggers accumulation of phosphatidylinositol 3',4'-bisphosphate (PtdIns-3,4-P2) (Gironcel, D. , Racaud-Sultan, C., Payrastre, B., Haricot, M., Borchert, G., Kieffer, N., Breton, M., and Chap, H. (1996) FEBS Lett. 389, 253-256). ADP has also been involved in platelet spreading. Therefore, in order to study a possible role of phosphoinositide 3-kinase in platelet morphological changes following adhesion, human platelets were pretreated with specific phosphoinositide 3-kinase inhibitors LY294002 and wortmannin. Under conditions where PtdIns-3, 4-P2 synthesis was totally inhibited (25 microM LY294002 or 100 nM wortmannin), platelets adhered to the fibrinogen matrix, extended pseudopodia, but did not spread. Moreover, addition of ADP to the medium did not reverse the inhibitory effects of phosphoinositide 3-kinase inhibitors on platelet spreading. Although synthetic dipalmitoyl PtdIns-3,4-P2 and dipalmitoyl phosphatidylinositol 3',4', 5'-trisphosphate restored only partially platelet spreading, phosphatidylinositol 4',5'-bisphosphate (PtdIns-4,5-P2) was able to trigger full spreading of wortmannin-treated adherent platelets. Following 32P labeling of intact platelets, the recovery of [32P]PtdIns-4,5-P2 in anti-talin immunoprecipitates from adherent platelets was found to be decreased upon treatment by wortmannin. These results suggest that the lipid products of phosphoinositide 3-kinase are required but not sufficient for ADP-induced spreading of adherent platelets and that PtdIns-4,5-P2 could be a downstream messenger of this signaling pathway.

Identification and cDNA cloning of a novel mammalian C2 domain-containing phosphoinositide 3-kinase, HsC2-PI3K

Phosphoinositide (PI) 3-kinases have been shown to have critical roles in signal transduction, cell transformation and intracellular protein trafficking. Reverse-transcription polymerase chain reaction methods, using degenerate primers derived from the lipid kinase consensus region, were utilised to identify PI 3-kinases in the normal human breast. Here we report the cDNA cloning of a novel human PI 3-kinase isoform, HsC2-PI3K. This PI 3-kinase is most closely related to the recently described C2 domain-containing family of PI 3-kinases which includes Drosophila PI3K_68D/cpk and murine cpk-m/p170. Sequence analysis suggests that HsC2-PI3K is a second distinct mammalian member of the C2 domain-containing PI 3-kinase family. Northern blot analysis of human tissues indicates that HsC2-PI3K is widely expressed. Fluorescence in situ hybridisation has mapped HsC2-PI3K to chromosome 1q32.

Cloning of cDNAs encoding two isoforms of 68-kDa type I phosphatidylinositol-4-phosphate 5-kinase

Accumulating evidence suggests that phosphatidylinositol metabolism is essential for membrane traffic in the cell. Of particular importance, phosphatidylinositol transfer protein and the type I phosphatidylinositol- 4-phosphate 5-kinase (PI4P5K) have been identified as cytosolic components required for ATP-dependent, Ca2+-activated secretion. In order to identify PI4P5K isoforms that may play important roles in regulated insulin secretion from pancreatic beta-cells, we employed the polymerase chain reaction with degenerate primers and screening of a cDNA library of the murine pancreatic beta-cell line MIN6. Two novel cDNAs, designated PI4P5K-Ialpha and PI4P5K-Ibeta, were identified, which contained complete coding sequences encoding 539- or 546-amino acid proteins, respectively. These cDNAs were expressed in mammalian cells with an adenoviral expression vector. Proteins of both isoforms migrated at 68 kDa on SDS-polyacrylamide gel electrophoresis and exhibited phosphatidylinositol-4-phosphate 5-kinase activity, which was activated by phosphatidic acid, indicating that these proteins were type I isoforms. While these isoforms share a marked amino acid sequence homology in their central portion, the amino- and carboxyl-terminal regions differ significantly. Northern blot analysis depicted that tissue distributions differed between the two isoforms. Molecular identification of type I PI4P5K isoforms in insulin-secreting cells should provide insights into the role of phosphatidylinositol metabolism in regulated exocytosis of insulin-containing large dense core vesicles.

A family of phosphoinositide 3-kinases in Drosophila identifies a new mediator of signal transduction

BACKGROUND

Mammalian phosphoinositide 3-kinases (PI 3-kinases) are involved in receptor-mediated signal transduction and have been implicated in processes such as transformation and mitogenesis through their role in elevating cellular phosphatidylinositol (3,4,5)-trisphosphate. Additionally, a PI 3-kinase activity which generates phosphatidylinositol 3-phosphate has been shown to be required for protein trafficking in yeast.

RESULTS

We have identified a family of three distinct PI 3-kinases in Drosophila, using an approach based on the polymerase chain reaction to amplify a region corresponding to the conserved catalytic domain of PI 3-kinases. One of these family members, PI3K_92D, is closely related to the prototypical PI 3-kinase, p110 alpha; PI3K_59F is homologous to Vps34p, whereas the third, PI3K_68D, is a novel PI 3-kinase which is widely expressed throughout the Drosophila life cycle. The PI3K_68D cDNA encodes a protein of 210 kDa, which lacks sequences implicated in linking p110 PI 3-kinases to p85 adaptor proteins, but contains an amino-terminal proline-rich sequence, which could bind to SH3 domains, and a carboxy-terminal C2 domain. Biochemical analyses demonstrate that PI3K_68D has a novel substrate specificity in vitro, restricted to phosphatidylinositol and phosphatidylinositol 4-phosphate, and is unable to phosphorylate phosphatidylinositol (4,5)-bisphosphate, the implied in vivo substrate for p110.

CONCLUSIONS

A family of PI 3-kinases in Drosophila, including a novel class represented by PI3K_68D, is described. PI3K_68D has the potential to bind to signalling molecules containing SH3 domains, lacks p85-adaptor-binding sequences, has a Ca(2+)-independent phospholipid-binding domain and displays a restricted in vitro substrate specificity, so it could define a novel signal transduction pathway.

Effects of growth factors on phosphatidylinositol-3 kinase in astroglial cells

Growth factors differently regulate astroglial cell differentiation and proliferation. In an effort to understand the early intracellular events promoted by growth factors in astroglial cells, we have determined the effects of insulin-like growth factor I (IGF1), insulin, platelet-derived growth factor (PDGF), epidermal growth factor (EGF) and fibroblast growth factors (FGFs) on phosphatidylinositol-3 kinase (PI(3)-kinase). In astroglial cells cultured in serum-free medium, IGF1, PDGF, and EGF, which stimulate cell proliferation, increased PI(3)-kinase activity immunoprecipitated with anti-phosphotyrosine antibodies as shown by thin layer chromatography and high performance liquid chromatography. FGFa and FGFb, which strongly stimulate proliferation, glutamine synthetase, and deiodinase activities and modify cell morphology, have no effect on PI(3)-kinase activity. Addition of 1 nM PDGF, 10 nM IGF1, or 100 nM EGF to the culture medium rapidly stimulated PI(3)-kinase activity which declined slowly after 2 min. The stimulation of PI(3)-kinase increased with growth factor concentration. The maximum increase in PI(3)-kinase activity occurred with 50 nM IGF1, 1 nM PDGF, or 100 nM EGF. Since insulin was active only at high concentration (1 microM), its effect was probably mediated through IGF1 receptors and not through insulin receptors. IGF1 and PDGF, to a lesser degree, also increased the PI(3)-kinase activity associated with pp60c-src protein. Immunoblots performed with an antibody directed against the p85-subunit of the PI(3)-kinase confirmed that IGF1 increased the number of PI(3)-kinase molecules associated with phosphotyrosine-containing proteins or with c-src protein. Each growth factor affects in a different manner the association of PI(3)-kinase with phosphotyrosine-containing proteins and with pp60c-src and thus probably modulates intracellular signals downstream of PI(3)-kinase in astroglial cells.

Phosphatidylinositol 3-kinase

Currently, a central question in biology is how signals from the cell surface modulate intracellular processes. In recent years phosphoinositides have been shown to play a key role in signal transduction. Two phosphoinositide pathways have been characterized, to date. In the canonical phosphoinositide turnover pathway, activation of phosphatidylinositol-specific phospholipase C results in the hydrolysis of phosphatidylinositol 4,5-bisphosphate and the generation of two second messengers, inositol 1,4,5-trisphosphate and diacylglycerol. The 3-phosphoinositide pathway involves protein-tyrosine kinase-mediated recruitment and activation of phosphatidylinositol 3-kinase, resulting in the production of phosphatidylinositol 3,4-bisphosphate and phosphatidylinositol 3,4,5-trisphosphate. The 3-phosphoinositides are not substrates of any known phospholipase C, are not components of the canonical phosphoinositide turnover pathway, and may themselves act as intracellular mediators. The 3-phosphoinositide pathway has been implicated in growth factor-dependent mitogenesis, membrane ruffling and glucose uptake. Furthermore the homology of the yeast vps34 with the mammalian phosphatidylinositol 3-kinase has suggested a role for this pathway in vesicular trafficking. In this review the different mechanisms employed by protein-tyrosine kinases to activate phosphatidylinositol 3-kinase, and its involvement in the signaling cascade initiated by tyrosine phosphorylation, are examined.

Phosphorylation of lipids in rat primary glial cells and immortalized astrocytes (DITNC)

Phosphatidylinositol 4,5-bisphosphate, a substrate in the signal transduction pathway involving phospholipase C, is synthesized in cell membranes by phosphorylation of phosphatidylinositol (PI) and phosphatidylinositol 4-phosphate (PI-4-P). Incubation with [gamma-32P]adenosine triphosphate of membranes isolated from primary glial cells in culture and from an immortalized astrocyte cell line (DITNC) and subsequent separation of the lipids by high-performance thin-layer chromatography revealed a number of labeled lipid bands. Further analysis of the deacylated products by high-performance liquid chromatography indicated the presence of two PI-4-P bands and a band corresponding to lysoPIP (PIP, PI phosphate). The two PI-4-P bands were more prominent in the samples from astrocyte cell membranes than from synaptic plasma membranes and plasma membranes. Analysis of the fatty acid composition of PI by gas-liquid chromatography revealed that both 20:3n-9 and 20:4n-6 were present in PI of cultured cell membranes, whereas the brain membranes contained exclusively 20:4n-6. The two PI-4-P bands in these membranes can be attributed to the presence of different molecular species of PI. Assignment of the fraction corresponding to lysoPIP was consistent with the observation that an increase in label in this lipid band occurred upon incubation of DITNC cell supernatant with lysoPI (1-stearoyl PI). This suggests that endogenous lysoPI present in the cell membranes can be phosphorylated to form lysoPIP.

Haematopoietic colony stimulating factors CSF-1 and GM-CSF increase phosphatidylinositol 3-kinase activity in murine bone marrow-derived macrophages

The activity of phosphatidylinositol (PI) 3-kinase was examined in murine bone marrow-derived macrophages (BMM) stimulated with the haematopoietic growth factors colony stimulating factor-1 (CSF-1) and granulocyte/macrophage-CSF (GM-CSF). PI 3-kinase was immunoprecipitated from cell lysates using anti-phosphotyrosine antibody or an antibody directed against the 85K subunit of PI 3-kinase, and the activity assayed by the phosphorylation of PI in the presence of [gamma 32P]-ATP. The results demonstrate that CSF-1 increases the activity of PI 3-kinase, as compared to the non-stimulated control, in murine macrophages. Maximum activity was seen after 10 min of stimulation with CSF-1 at 3000-5000 U/ml. The dose-response of CSF-1 is consistent with other biochemical effects of CSF-1 seen in the BMM. GM-CSF also stimulated PI 3-kinase activity although to a lesser extent than CSF-1, correlating well with their degree of mitogenic activity on the BMM. Non-mitogenic macrophage activating agents, such as the phorbol myristate acetate, lipopolysaccharide, concanavalin A and formyl-methionyl-leucyl-phenylalanine, did not significantly increase the PI 3-kinase activity. Furthermore, CSF-1 failed to stimulate PI 3-kinase activity in resident peritoneal macrophages, a population of macrophages with poor proliferative capacity. These results suggest that the PI 3-kinase activity may be involved in the haemopoietic growth factor signalling pathways regulating macrophage growth.

Activation of phosphatidylinositol 3-kinase and phosphatidylinositol 4-kinase during rat parotid acinar cell proliferation

We have recently shown that beta-adrenergic agonist, isoproterenol-induced parotid acinar cell proliferation is in part mediated by elevated levels of surface galactosyltransferase which undergoes interaction with the EGF-R. The receptor subsequently undergoes autophosphorylation on the tyrosine residues in a manner similar to its 'receptor-ligand' interaction (Purushotham et al. (1992) Biochem. J. 284, 767-776). In this study, we provide evidence for phosphatidylinositol 3-kinase and 4-kinase as cytoplasmic signalling proteins involved in both the isoproterenol and EGF-stimulated signal transduction upon in vitro and in-vivo stimulation of parotid acinar cells. Total cell lysate activity for the PtdIns 4-kinase was 2- and 3-fold higher than unstimulated control cells, while the PtdIns 3-kinase was 1.4- and 2.8-fold higher following stimulation by isoproterenol or EGF, respectively. Increases of 6- and 2-fold in phosphatidylinositol 3-kinase were observed in anti-phosphotyrosine-antibody-immunoprecipitated cell lysates upon in-vitro growth stimulation with isoproterenol or EGF, respectively. There was an increase in tyrosine phosphorylation of the holoenzyme and association of the p85 subunit of phosphatidylinositol 3-kinase with EGF-R in response to both isoproterenol and EGF treatments. This corresponded with the mobilization of p85 from the cytoplasm to the plasma membrane upon growth stimulation. These results further implicate the phosphoinositide metabolites in the second messenger signalling pathways of isoproterenol-induced rat parotid cell proliferation. The parallel utilization of EGF indicate that the post-transductional mechanisms of isoproterenol-induced acinar cell proliferation are similar to the growth-factor-mediated activation of intracellular signalling pathways for cell growth.

Insulin stimulates phosphatidylinositol-3-kinase activity in rat adipocytes

Phosphatidylinositol (PtdIns) 3-kinase is thought to participate in the signal transduction pathways initiated by the activation of receptor tyrosine kinases including the insulin receptor. To approach the physiological relevance of this enzyme in insulin signaling, we studied the activation of PtdIns-3-kinase in adipocytes, a major insulin target tissue for glucose transport and utilisation. To analyze possible interactions of the enzyme with cellular proteins, immunoprecipitations with the following antibodies were performed: (a) anti-phosphotyrosine antibodies, (b) two antibodies to the 85-kDa subunit of PtdIns-3-kinase (p85) and (c) an antibody to the 185-kDa major insulin receptor substrate (p185). We show that in cell extracts from adipocytes exposed to insulin, and after immunoprecipitation with an anti-phosphotyrosine antibody and an antibody to p85, we are able to detect a PtdIns-3-kinase activity stimulated by the hormone. Similarly, after immunoprecipitation with an antibody to p185, an increase in the PtdIns-3-kinase activity could be demonstrated. Taken together these results suggest that, upon insulin stimulation of fat cells, PtdIns-3-kinase itself is tyrosine phosphorylated and/or associated with an insulin receptor substrate, such as p185, which could function as a link between the insulin receptor and PtdIns-3-kinase. The PtdIns-3-kinase was activated within 1 min of exposure to insulin, and the half-maximal effect was reached at the same concentration, i.e. 3 nM, as for stimulation of the insulin receptor kinase. Subcellular fractionation showed that PtdIns-3-kinase activity was found both in the membranes and in the cytosol. Further, immunoprecipitation with an antibody to p85, which possesses the capacity to activate PtdIns-3-kinase, suggests that the presence of the enzyme in the membrane may be due to an insulin-induced recruitment of the PtdIns-3-kinase from the cytosol to the membrane. Finally, we used isoproterenol, which exerts antagonistic effects on insulin action. This drug was found to inhibit both the PtdIns-3-kinase and the insulin receptor activation by insulin, suggesting that the activation of the PtdIns-3-kinase was closely regulated by the insulin receptor tyrosine kinase. The occurrence of an insulin-stimulated PtdIns-3-kinase in adipocytes leads us to propose that this enzyme might be implicated in the generation of metabolic responses induced by insulin.

Calcium homeostasis in Trypanosoma cruzi amastigotes: presence of inositol phosphates and lack of an inositol 1,4,5-trisphosphate-sensitive calcium pool

The permeabilization of Trypanosoma cruzi amastigotes with digitonin allowed the study of Ca2+ fluxes between intracellular organelles in situ. In addition, fura-2 was used to determine the cytosolic Ca2+ concentration in the intact cells. When amastigotes were permeabilized in a reaction medium containing MgATP, succinate and 3.5 microM Ca2+, they lowered the medium Ca2+ concentration to the submicromolar level, a range which correlates favorably with that detected in the intact cells with fura-2. The presence of 1 microM FCCP strongly decreased the initial rate of Ca2+ sequestration by these permeabilized cells. This FCCP-insensitive Ca2+ uptake, probably represented by the endoplasmic reticulum, was completely inhibited by 500 microM vanadate. On the other hand, when vanadate instead of FCCP was present, the initial rate of Ca2+ accumulation was decreased and the Ca2+ set point was increased to about 0.8 microM. The succinate dependence and FCCP sensitivity of the later Ca2+ uptake indicate that it may be exerted by the mitochondria. Despite the presence of inositol phosphates, as determined by [3H]inositol incorporation, and of a large extramitochondrial Ca2+ pool, no IP3-sensitive or thapsigargin-sensitive Ca2+ release could be detected in either amastigotes or epimastigotes.

Antigen Responsive Antibody–Receptor Kinase Chimera

We have constructed chimeric receptors, combining murine IgM and the cytoplasmic portion of human epidermal growth factor receptor (EGFR), with the aim of developing a novel immunosensor with antigen-dependent phosphorylation activity. When intact IgM was used, the chimeric receptor showed both antigen binding and protein tyrosine kinase activity, but the kinase activity was constitutive and independent of antigen binding. However, with IgM lacking the CH2 domain, the autophosphorylation activity increased with increasing concentrations of anti-IgM or hapten-BSA conjugate. Monovalent hapten could not induce phosphorylation but inhibited stimulation by hapten-conjugated BSA.

The effect of orthovanadate on phosphoinositide metabolism in NIH 3T3 fibroblasts

Orthovanadate is an agent known to stimulate cell growth and mimic insulin action. The effects of this compound on phosphoinositides in NIH 3T3 cells were examined. Both 100 and 1000 microM orthovanadate were found to increase the cellular content of inositol phosphate secondary to the activation of phosphatidylinositol-specific phospholipase C (PtdIns-PLC). The time course, dependence on orthovanadate concentration, and sensitivity to the isoflavone genistein were similar for orthovanadate-induced accumulation of inositol phosphate and protein tyrosine phosphate, indicating that there is a correlation between cellular protein tyrosine phosphate levels and PtdIns-PLC activity. Increased phosphatidylinositol phosphate (PtdInsP) content also occurred when cells were incubated with orthovanadate and appeared to result from the activation of PtdIns kinase. This effect was not correlated with cellular protein tyrosine phosphate content. Hence, orthovanadate is shown to affect phosphoinositide metabolism at a minimum of two sites by both tyrosine phosphate-dependent and -independent mechanisms.

Partial purification and characterization of two forms of phosphatidylinositol 4-phosphate 5-kinase from human platelet membrane

The phosphatidylinositol 4-phosphate 5-kinase (PIP kinase) was isolated from the cholate extract of human platelet membranes. Two major activity peaks (PIP kinase I and PIP kinase II) were resolved by successive chromatographies on Fast Q-Sepharose, heparin-Sepharose, Mono Q and heparin-agarose columns. The PIP kinase I appears to be distinct from the PIP kinase II with regard to Mr (51 kDa and 47 kDa as determined by SDS-PAGE). The two forms of PIP kinase showed similarity in Km for ATP and Mg2+ dependency, but some differences were observed in effects of Mn2+ and phosphatidylethanolamine on the activity.

Hepatic phosphatidylinositol kinase activity in continuously endotoxemic rats

The activity of phosphatidylinositol (PI) kinase and the content and fatty acid composition of inositol phospholipids (IPLs) were analyzed in the livers of rats that had been continuously infused with Escherichia coli endotoxin (ET) or saline for 30 h. Maximal enzymatic activity in total liver membrane fractions was observed in the presence of 1 mM ATP, 20 mM MgCl2, exogenously added 0.3 mM PI and Triton X-100 (0.25%). The activity of PI kinase for endogenous and exogenous PI was 43 and 79% higher respectively, in ET- as compared with saline-infused rats. The Km of the enzyme for ATP was not altered (0.175 mM), while the apparent Vmax was higher for ET- as compared with saline-infused rats (0.48 and 0.38 nmol of phosphatidylinositol 4-phosphate formed/mg protein per min, respectively). The ET-induced higher activity of PI kinase was paralleled by a 68-78% increase in the content of polyphosphoinositides (PPI), while PI content was unchanged. All IPLs from livers of endotoxemic rats had a lower content of arachidonic acid. We demonstrate for the first time that ET can directly and/or indirectly stimulate the net synthesis of PPI in liver cells. This effect could serve to modulate the PPI derived signals by increasing the availability of the substrate phosphatidylinositol 4,5-bisphosphate.

Pathway of phosphatidylinositol(3,4,5)-trisphosphate synthesis in activated neutrophils

Neutrophils activated by the formyl peptide f-Met-Leu-Phe transiently accumulate a small subset of highly polar inositol lipids. A similar family of lipids also appear in many other cells in response to a range of growth factors and activated oncogenes, and are presumed to be the direct or indirect products of 3-phosphatidylinositol kinase. The structures of these lipids are shown to be phosphatidylinositol 3-phosphate, phosphatidylinositol-(3,4)bisphosphate and phosphatidylinositol-(3,4,5)trisphosphate, and we present evidence that in intact neutrophils a phosphatidyl-inositol-(4,5)bisphosphate-3-kinase seems to be the focal point through which agonists stimulate the formation of 3-phosphorylated inositol lipids.

Thrombin stimulates the production of phosphatidylinositol 3,4-bisphosphate in human erythroleukemia cells

The human erythroleukemic cell line, HEL, which has numerous platelet markers, shows enhanced inositol phosphate production in response to thrombin. We investigated the production of phosphoinositides in HEL cells and showed that thrombin stimulates the turnover of several phosphoinositides including the synthesis of phosphatidylinositol 3,4-bisphosphate (PtdIns(3,4)P2). Phosphatidylinositol 3-monophosphate is also produced in HEL cells and its synthesis is not stimulated by thrombin. Pretreatment of HEL cells with the stable prostacyclin analog iloprost inhibits the thrombin-induced increase in the production of PtdIns(3,4)P2. 3-Phosphorylated phosphoinositides have been implicated in signal transduction and regulation of cell proliferation in other cells and may be involved in signal transduction in HEL cells.

Anti-phosphotyrosine immunoprecipitation of phosphatidylinositol 3' kinase activity in different cell types after exposure to epidermal growth factor

In previous studies from this laboratory, it was shown that mouse epidermal growth factor (mEGF) or insulin increased the labeling of phosphaditylinositol-3,4-bisphosphate (PI-3,4-P2) in MA-10 cells prelabeled with different radioactive precursors (Pignataro, O.P., and Ascoli, M. (1990) J. Biol. Chem. 265, 1718-1723 and Mol. Endocrinol. (1990) 4, 758-765). In order to further characterize this phenomenon we sought to determine if we could use anti-phosphotyrosine antibodies to immunoprecipitate a phosphatidylinositol (PI) kinase activity from MA-10 cells treated with mEGF or insulin. Our data indicate that this is indeed the case, and that the PI kinase precipitated is a PI-3' kinase. A second cell type, A431 cells, in which we were unable to detect an increase in PI-3,4-P2 labeling when stimulated with mEGF or insulin, was also studied. It was found that, as in MA-10 cells, A431 cells also contain an immunoprecipitable PI-3' kinase activity that is increased in response to mEGF or insulin.

Resistance to receptor-mediated degradation of a murine epidermal growth factor analogue (EGF-Val-47) potentiates its mitogenic activity

In most cell types two classes of epidermal growth factor (EGF) receptors can be found: a major class that binds EGF with relatively low affinity and a minor class that binds with very high affinity. Structure-function studies have shown that mutations at amino acid 47 in the EGF molecule severely reduce its affinity for the EGF receptor but do not cause preferential binding to one or the other subclass of receptors. Using three EGF derivatives with a mutation at amino acid 47 (Ser-47, Leu-37-Tyr-47, and Val-47), we have investigated the relative contribution of the two receptor subclasses to the EGF-dependent mitogenic response. We show that mitogenicity correlates exclusively with occupancy of the high-affinity receptor and that full occupancy of this subclass is required for maximal stimulation. In addition we demonstrate that for the EGF-Val-47 analogue this requirement can be abrogated and half-maximal biological activity reached with a high-affinity receptor occupancy of only 8%. While the rate of internalization did not significantly differ between EGF-Val-47 and native mEGF, the analogue was much more resistant to degradation by cellular proteases and, after binding and receptor-mediated internalization, was released into the medium predominantly in an intact form. We propose that the increased mitogenicity of EGF-Val-47 is due to its prolonged half-life, resulting in continued occupancy of the high-affinity EGF receptor.

myo-inositol metabolites as cellular signals

The discovery of the second-messenger functions of inositol 1,4,5-trisphosphate and diacylglycerol, the products of hormone-stimulated inositol phospholipid hydrolysis, marked a turning point in studies of hormone function. This review focuses on the myo-inositol moiety which is involved in an increasingly complex network of metabolic interconversions, myo-Inositol metabolites identified in eukaryotic cells include at least six glycerophospholipid isomers and some 25 distinct inositol phosphates which differ in the number and distribution of phosphate groups around the inositol ring. This apparent complexity can be simplified by assigning groups of myo-inositol metabolites to distinct functional compartments. For example, the phosphatidylinositol 4-kinase pathway functions to generate inositol phospholipids that are substrates for hormone-sensitive forms of inositol-phospholipid phospholipase C, whilst the newly discovered phosphatidylinositol 3-kinase pathway generates lipids that are resistant to such enzymes and may function directly as novel mitogenic signals. Inositol phosphate metabolism functions to terminate the second-messenger activity of inositol 1,4,5-trisphosphate, to recycle the latter's myo-inositol moiety and, perhaps, to generate additional signal molecules such as inositol 1,3,4,5-tetrakisphosphate, inositol pentakisphosphate and inositol hexakisphosphate. In addition to providing a more complete picture of the pathways of myo-inositol metabolism, recent studies have made rapid progress in understanding the molecular basis underlying hormonal stimulation of inositol-phospholipid-specific phospholipase C and inositol 1,4,5-trisphosphate-mediated Ca2+ mobilisation.

Partial purification and characterization of phosphatidylinositol kinases from human platelets

Most of human platelet phosphatidylinositol (PI) kinase activity (approx. 80%) was associated with the membrane fraction and its majority was released by the extraction with Triton X-100 after KCl treatment. Two major activity peaks (mPIK-I and mPIK-III) were obtained by Mono Q column chromatography. They were distinct from each other with regard to Mr (76,000 and 80,000 as determined by gel-filtration chromatography), apparent Km values for ATP, effect of arachidonic acid and phosphatidylserine and detergent requirement. Triton X-100 inhibited the activity of mPIK-I but rather weakly enhanced the mPIK-III activity, and sodium cholate remarkably inhibited both mPIK-I and mPIK-III activities. Their products were identified to be phosphatidylinositol 4-phosphate. On the other hand, about 20% of PI kinase activity was recovered from the cytosolic fraction and two activity peaks (cPIK-I and cPIK-II) were resolved on Mono Q column chromatography. There were no significant differences in biochemical properties between cPIK-I and cPIK-II. Both of them had Mr approx. 550,000 as determined by gel-filtration chromatography and were activated by sodium cholate to a greater extent than by Triton X-100. The results suggest that the major PI kinases (mPIK-I and mPIK-III) are PI 4-kinase and mPIK-I is distinct from PI 4-kinases in other sources especially with regard to the effect of Triton X-100.